This is cool. When does a planet look like a comet? When it sports a long sodium tail, as seen in this recent capture on the front page of today's Space Weather, courtesy of Sebastian Voltmer: spaceweather.com - Voltmer used a special sodium filter to capture the elusive sodium tail of Mercury. Suspected since the 1980s, the tail was finally confirmed in 2001, and seen upclose via NASA's Mercury Messenger spacecraft. The tail reaches maximum luminosity over the next week.